1. Field of the Invention
This invention relates to an ink-jet cartridge for regulating the ink pressure within the ink reservoir of an ink-jet pen. The cartridge automatically controls the pressure difference inside and outside to prevent ink from leakage.
2. Related Art
In ink-jet printing, the common types of ink-jet control are heat bubble and piezoelectric pressure wave methods. A heat bubble type print head includes a thin-film resistor that is heated to cause sudden vaporization of a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through a print head orifice to a printing paper. Although conventional print heads are effective for ejecting ink drops from a reservoir, they need extra mechanisms for preventing ink from permeating through the print head when the print head is inactive. The common mechanism provides a slight back pressure at the print head to prevent ink leakage from the print head whenever the print head is inactive. The term "back pressure" used herein means the partial vacuum within the reservoir that resists the flow of ink through the print head. Back pressure is considered in the positive sense so that an increase in back pressure represents an increase the partial vacuum.
In the design of an ink-jet cartridge, the following requirements should be considered:
a) the back pressure at the print head must be at all times strong enough for preventing ink leakage. But the back pressure must not be so strong that the print head is unable to overcome the back pressure to eject ink droplets, or the size of the ink droplets and the print quality are influenced; PA1 b) the back pressure at the print head must be varied according to the pressure of environment and maintained in a suitable range, for example, when the pressure of environment drops, the back pressure must be relatively higher to prevent ink leakage; and PA1 c) the operating effect of the ink reservoir also influences the back pressure at the ink-jet cartridge. That is, the back pressure raises when the ink is used up, therefore, it must be suitably regulated to assure the ink ejection. Without regulation of this back pressure increase, the ink-jet pen will eventually fail because the print head will be unable to overcome the increased back pressure to eject ink drops. PA1 an ink reservoir for storing ink and providing ink for ejection, the reservoir includes: PA1 a rigid body, formed with rigid material, for storing ink; PA1 a vent, located on top of the rigid body, communicated with the ambient air, for adjusting the air pressure inside the ink reservoir; PA1 a print head for ejecting ink and printing; PA1 a print ink chamber for preparing ink ready for the print head, the ink level of the chamber is normally lower than the ink level of the rigid body; and PA1 a back pressure chamber, having one end fluid-communicated with the ink in the rigid body, and the other end fluid communicated with the print ink chamber via an overflow vent which is at a position higher than the ink level of the rigid body, a suitable back pressure is applied to the back pressure chamber and the print ink chamber so that, when printing, the increased back pressure at the print ink chamber sucks ink from the ink reservoir through the overflow vent into the print ink chamber; while, when not printing, the extra height of the ink in the back pressure chamber relative to the ink level of the ink reservoir balances the back pressure at the back pressure chamber and the print ink chamber to prevent ink leakage from the print head.
In the current arts, the back pressure of the ink reservoir is regulated by an accumulator. The accumulator is generally an elastic bag varying its volume between a minimum volume position and a maximum volume position to regulate the ink reservoir volume and adjust the back pressure so that the back pressure remains within an operating range that is suitable for preventing ink leakage while permitting the print head to continue ejecting ink droplets.
As shown in FIG. 1, an accumulator 20 for an ink-jet cartridge 1 is disclosed by U.S. Pat. No. 5,409,134. The ink-jet cartridge 1 includes a reservoir 10 which has a rigid body 11 and a print head 30 located at the bottom of the reservoir 10 for providing ink 40 to the print media through a print head 30. An accumulator 20 mounted inside the reservoir 10 and includes mainly a spring 21a, 21b; an expandable bag 22a, 22b which is formed of two thin flexible sheets 23, 24. The bag 22a, 22b is fastened via a fitment 25 having pins 27 and release patch 26 to the top of the rigid body 11, and a breather strip 28 fixed between the flexible sheets 23, 24 so that the interior of the bag 22a, 22b is in fluid communication with the ambient air through a duct 12. With the accumulator 20 in place, the reservoir 10 is filled with ink 40 through an unshown sealable port. A slight back pressure is established within the reservoir 10. The slight back pressure is the minimum amount of back pressure necessary to keep ink from leaking through the print head 30 when the print head is inactive.
As the cartridge 1 is used for printing, the air pressure within the reservoir 10 decreases (hence, the back pressure increases) as ink 40 is depleted. During printing, the bag 22a, 22b expands as a result of the back pressure increase. The bag expansion decreases the volume of the reservoir 10 to maintain the reservoir back pressure within a range such that the print head 30 is able to continue ejecting ink 40 from the reservoir 10. If the ambient pressure should thereafter decrease (for example, during air transport of the cartridge), the bag 22a, 22b will contract to increase the reservoir volume so that the back pressure within the reservoir 10, relative to ambient, does not drop to a level that permits ink 40 to leak from the print head 30.